6 Message Formats

MiLLennium Command Descriptions Manual 37

positioning algorithm would be used. This is for a system in which both the reference and remote stations utilize
NovAtel receivers.
Note that the error-detection capability of an RTCM-format message is less than that of an RTCA-format mess age.
The communications equipment that you use may have an error-detection capability of its own to supplement that
of the RTCM message, although at a penalty of a higher overhead (see the discussion at the beginning of this
chapter). Consult the vendor’s documentation for further information.
• RTCM Type 3 Reference Station Position
A Type 3 message contains reference station position information. This message must be sent at least once every
30 seconds, although it is recommended that it be sent once every 10 seconds. It uses four RTCM data words
following the two-word header, for a total frame length of six 30-bit words (180 bits).
• RTCM Type 59 NovAtel Proprietary (RT-20)
A Type 59N message contains reference station satellite observation information, and should be sent once every 2
seconds. It is variable in size, and can be up to thirty three 30-bit words (990 bits) long.
If RTCM-format messaging is being used, the optional station id field that is entered using the FIX POSITION
command can be any number within the range of 0 - 1023 (e.g. 119). The representation in the log message would
be identical to what was entered.
RTCM STANDARD COMMANDS and LOGS
The Global Positioning System is a world-wide positioning service developed by the U.S. Department of Defense
(DOD) and is operated and maintained by the U.S. Air Force Space Division. As usage of the GPS Standard
Positioning Service (SPS) has gained world wide commercial acceptance, the applications have become wide and
varied. Of special importance have been the developments in the use of differential GPS (DGPS). DGPS enables
system users to leap from nominal 100 metre system accuracies (single point) to the more desirable one to five
metre nominal accuracies possible from utilizing differential corrections between reference and rem ote stations.
As DGPS systems exist all over the world, the need arose to establish a set of operating standards that all DGPS
receivers could use for the purpose of transmitting and receiving differential corrections between GPS receivers of
various types, regardless of receiver design or manufacturer.
The Radio Technical Commission for Maritime Services (RTCM) was established to facilitate the establishment of
various radio navigation standards, which includes recommended GPS differential standard formats.
The standards recommended by the Radio Technical Commission for Maritime Services Special Committee 104,
Differential GPS Service (RTCM SC-104,Washington, D.C.), have been adopted by NovAtel for implementation
into the GPSCard. Because the GPSCard is capable of utilizing RTCM formats, it can easily be integrated into
positioning systems around the globe.
As it is beyond the scope of this manual to provide in-depth descriptions of the RTCM data formats, it is
recommended that anyone requiring explicit descriptions of such, should obtain a copy of the published RTCM
specifications. See Appendix F for reference information.
2. For further information on RTCM SC-104 messages, you may wish to refer to:
RTCM Recommended Standards for Differential Navstar GPS Service, Version 2.1, RTCM Paper 194-93/SC104-
STD (January 3, 1994)